This invention relates to a radio communication network for communicating data from a plurality of remote data generating stations to a central station where the data is collated and analyzed.
Devices of this type are particularly but not exclusively useful in communicating from remote meter reading devices of a utility metering system. The present invention will be described in relation to a remote or automatic meter reading system, however it will be appreciated that alternative uses of the network are possible. The device of the present invention may however be used advantageously in many other remote metering or monitoring systems in which its is necessary for a large number of remote stations to communicate to a central station, for example, burglar alarm systems or industrial process monitoring systems.
Many attempts have been made in recent years to develop an automatic meter reading system for utility meters which avoids the necessity for meter reading personnel to inspect the meter and to physically note the meter readings. There are of course many reasons for attempting to develop a system of this type which will not be expanded at length here.
However most of these systems have achieved little success. The system which has achieved most success or is most widely used is a system in which the automatic meter reading unit is mounted on an existing meter at the usage site and includes a relatively small transmitter and receiver unit of very short range. The unit is then polled on a regular basis by a travelling reading unit which is in practical terms carried around the various locations on a suitable vehicle and operated to roll each automatic meter reading unit in turn to obtain the stored data. This device is of course of only very limited value in that it is still necessary to transport the equipment around the various locations and hence only very infrequent, for example monthly, readings can be made. Basically this system only avoids the necessity of the meter reader actually entering the premises to physically inspect the meter which is of itself of some value but only limited value.
Alternative proposals in which reading from a central location is carried out have been made but have achieved little success. One proposal involves an arrangement in which communication is carried out using the power transmission line of the electrical utility. Communication is therefore carried out along the line and rolls each remote reading unit in turn. This device has encountered significant technical difficulties.
Other alternatives have attempted to use the pre-existing telephone lines for communication but this proposal has the significant disadvantage that it must involve a number of other parties, in particular the telephone company, in the implementation of the system and there is a significant reluctance by the utility companies to become involved in a system which cannot be entirely controlled and managed by themselves.
A yet further system using radio communication has been developed by Data Beam which was a subsidiary of Connecticut Natural Gas. This arrangement was developed approximately in 1986 and has subsequently received little attention and it is believed that no installations are presently operative.
In this arrangement the system includes a meter reading device mounted upon the meter concerned together with a transmitting antenna which is separate from the meter reading device and located on the building or other part of the installation site which enables the antenna to transmit over a relatively large distance The system is further provided with a number of receiving units with each arranged to receive data from a large number of transmitters stated to lie in the range 10,000 to 30,000. It is believed that the transmitters in order to achieve maximum range were intended to be positioned to some extent directionally or at least on a suitable position of the building to transmit to the intended receiving station. This arrangement leads to the use of a minimum number of receiving stations which it is believed was assumed to be the optimum technique for cost efficiency.
However the separate transmitter antenna generated significant installation problems in view of the necessity to wire the device through the building to a suitable location. In addition the high level of power used in transmission involved very expensive battery systems or very expensive mains wiring. The proposal to reduce this excessive cost was to use the transmission unit for a number of the different utilities used in the building so that the cost of the transmitter could be spread for example between three utilities supplied to the building. However this has the problem that it is necessary for the separate utility companies to cooperate in the installation. While this might therefore be highly desirable it is difficult to achieve on a practical basis.
In order to avoid timing problems, the meter reading units were arranged to communicate on a pseudorandom time basis. However the very large number of meter reading units reporting to a single receiving station, up to 30,000, leads to a very high number of possible collisions between the pseudorandomly transmitted signals. The system therefore as proposed, with daily or more often reporting signals could lose as many as 20% to 50% of the signals transmitted due to collisions or interference which leads of course to a very low efficiency of data communication.
Furthermore, the use of transmitters at the meter reading units which are of maximum power leads to the necessity for a larger interference protection radius between systems using the same allocated frequency.
An alternative radio transmission network is known as ALOHA. In this system a number of broadcasting stations communicate with a single receiving station with the broadcasting stations transmitting at pseudorandom intervals. In this system therefore again, collisions will occur so that messages will be lost. The solution for this problem in the ALOHA system is for each of the broadcasting stations to monitor the retransmission of the information from the receiving station so that each broadcasting station is aware when its transmission has been lost. It is then programmed to retransmit the lost information after a predetermined generally pseudorandom period of time. However this system requires retransmission of the information from the receiving station to take place substantially immediately and requires each broadcasting station to also have a receiving capability. Cellular telephone networks have recently become implemented on a wide scale. These systems however use and allocate different frequencies to different remote stations. While this is acceptable in a high cost field of telecommunications, the costs and complications cannot be accepted in the relatively lower margin use of remote station monitoring. The technology of cellular telephones however leads to the perception in the art that devices of this type must use different frequency networks.
While theoretically automatic meter reading is highly desirable, it is of course highly price sensitive and hence it is most important for any system to be adopted for the price per unit of particularly the large number of meter reading units to be kept to a minimum. The high cost of high power transmission devices, receiving devices and battery systems generally leads to a per unit cost which is unacceptably high.